iommu/omap: Use BIT(x) macros in omap-iommu.h

Switch to using the BIT(x) macros in omap-iommu.h where
possible. This eliminates the following checkpatch check
warning:
    "CHECK: Prefer using the BIT macro"

A couple of the warnings were ignored for better readability
of the bit-shift for the different values.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>